There are ten rows of these tuberculated plates; but as they are disposed in pairs, each row of large pieces being united by a zigzag suture with another of a similar description, there are in reality only five large segments of the shell, each supporting a double row of tubercles.

1. Shell of Cidaris denuded of its spines. 2. A spine articulated with its corresponding tubercle.

Fig. 94. 1. Shell of Cidaris denuded of its spines. 2. A spine articulated with its corresponding tubercle: a, section of tubercle; b b, capsular ligament; c, base of spine.

(502). The reader must not, however, conclude that the great central tubercles above mentioned are the only parts of the shell to which spines are affixed; hundreds of smaller elevations are disseminated over the surface, whereunto smaller spicula are appended - although, from their diminutive size, these are of secondary importance in locomotion.

(503). The five large double segments that thus form the greater portion of the calcareous shell are separated from each other by the interposition of ten rows of perforated plates, likewise disposed in pairs, and composed of much smaller pieces than those which support the tubercles; hundreds of foramina, piercing these ambulacral bands, give passage to as many tubular feet or protrusible suckers, in every respect resembling those of Asterias, and distended by a similar apparatus.

(504). It is impossible, by any verbal description at all commensurate with the limits of our present undertaking, adequately to explain the more minute contrivances visible in the disposition of every portion of these wonderfully-constructed coverings: it is sufficient for our present purpose to observe that the globular crust of an Echinus is made up of several hundred polygonal pieces, of different sizes, and, although presenting every variety of outline, generally approximating more or less to a pentagonal form; that these pieces are so accurately and completely fitted to each other, that the lines uniting them are scarcely to be distinguished, even upon the most minute examination; and that from the union of so many distinct and dissimilar plates results a firm, compact, and beautiful box, similar to that represented in the figure. The first question that naturally suggests itself, on examining a shell of this description, is concerning the object to be attained by such remarkable complexity; it would appear, indeed, at first sight, that a simple calcareous crust, had it been allowed to exude from the entire surface of the Echinus, would gradually have moulded itself upon the body of the creature, and thus have formed a globular shell without suture, answering every purpose connected either with support or defence.

(505). A very little investigation, however, will suffice to show the necessity for the elaborate arrangement to which we have alluded. In the first place, as we shall immediately see, the earthy matter is not deposited upon the surface of the body, but within the soft external integument whereby it is secreted, - the interior of the shell being filled with sea-water, in which the viscera are loosely suspended. But a second and more important reason for the employment of so many pieces in the construction of the shell of an Echinus is to be derived from examining the mode in which the animal grows. Were it to retain the same dimensions throughout the whole period of its life, or could it, at stated intervals, cast off its old investment and secrete a new and more capacious covering as growth rendered the change necessary, a simple earthy crust would have been sufficient, without the presence of such an immense number of sutures and joinings. The calcareous plates of the Echinus, it must be remembered, are merely secreted from the soft parts, having no vital action going on within them whereby, as in the bones forming the skeletons of vertebrate animals, a continual deposition of fresh particles could be effected, allowing of extension by interstitial deposit.

How, therefore, could the growth of the Echinus be provided for? How is the gradual expansion of the entire shell, thus composed of a dense and extravascular crust, to be effected - and that without ever deranging the proportions of the whole fabric, or necessitating a loosening of its parts? No other contrivance could apparently have been adequate to the purpose: nevertheless we see how admirably, by the structure adopted, the growth of these creatures proceeds in all directions; for the living and vascular membrane that covers the whole external surface of the body dips down between the edges of the various calcareous pieces, and continually deposits, around the margin of each, successive layers of earthy particles, which, assuming a semi-crystalline arrangement, progressively increase the dimensions of each individual plate. But the continual augmentation in size which is thus going on is attended with no change in the mathematical figure of any given piece of the skeleton; so that, as they still increase in diameter by the unceasing deposition of earthy matter around the circumference of every plate, the spherical shell gradually expands, without in any degree altering its form or relative proportions, until it has acquired the mature dimensions belonging to its species.

(506). The tubular suckers or retractile feet, that are protruded at the pleasure of the animal from the countless minute apertures seen in the ten rows of ambulacral plates, are so similar in all essential points to those of Asterias already described, that little further need be said concerning their structure, or the mechanism whereby their motions are effected. The tubular part of each foot communicates with the interior of the shell by two branches passing through two apertures; and these branches, in some species (as Echinus saxatilis), receive offsets from the vessels that run along the centre of each ambulacral groove, and convey to the feet the fluid by which their distention is effected. In Echinus esculentus the feet open into a plexus of vessels, formed in leaf-like membranes, equal in number with the feet, and disposed in double rows upon the inner surface of the ambulacral pieces*, by the intervention of which they are connected with the canals above men-tioned.